There's not much to it, you just take the small 12V sealed lead-acid cell out from the bottom of the UPS, extend the two leads, and connect a larger capacity lead-acid battery of the same voltage.

If you don't recognize the terms "sealed", "lead-acid", "battery", "capacity", or "voltage" then you shouldn't do this.

About the only advantage of it is that it's cheap (free if you find a UPS in the trash with an already dead battery), but those cheap UPSs make really crap quality power, and for some of them the only reason they don't overheat is because their stock battery is so small. It's a bit like how you can cook a whole turkey in the microwave, but you probably don't want to.